1. Field of the Invention
The present invention relates generally to an anti-skid braking system for an automotive vehicle, capable of preventing an excessive slip ratio of the wheels of the vehicle upon brake application, and more particularly to such an anti-skid braking system for a four-wheel drive vehicle. The invention is also concerned with a method of controlling such an anti-skid braking system.
2. Discussion of the Prior Art
Skidding of the wheels of an automobile upon brake application will deteriorate the straight-line running stability and the braking effect. To avoid the skidding of the wheels, various approaches have been attempted. One of the approaches has been to incorporate a so-called "anti-skid" device in the braking system of the vehicle. The anti-skid device includes an actuator for increasing or decreasing the hydraulic braking pressures in the brake cylinders to apply brakes to the rotating wheels, and a controller to control the actuator. The controller is adapted to obtain the slip ratios of the wheels based on detected angular speeds or accelerations of the wheels, and commands the actuator so that the slip ratio of each wheel is held within an optimum range. An example of a widely used anti-skid braking system is disclosed in Laid-Open Publication No. 58-26660 of Japanese Patent Application. In the disclosed anti-skid braking system, the actuator has two separate hydraulic circuits for controlling the braking pressures for the left and right front wheels, independently of each other, and a single common hydraulic circuit for the left and right rear wheels. Also known is an anti-skid braking system wherein the actuator has two separate hydraulic circuits for the left and right rear wheels, as well as two separate hydraulic circuits for the front wheels.